1. Field of the Invention
The present invention is related to a map displaying method and apparatus, and a navigation system having such a map displaying apparatus. Particularly, the present invention is related to a stereoscopic map displaying method beneficial for the navigation system in which map information is selected from a map database according to a current location of a mobile object such as an automobile, and a perspective map viewing the ground surface along a direction angled to the ground surface is displayed.
2. Description of Related Art
The navigation system is known as an apparatus which assists a driver of an automobile by fetching digital map data stored in storage media such as CD-ROM, displaying a map showing buildings, roads and the like on a display screen, and superposing a current location of the automobile and a direction toward its destination on the map, which are detected by various sensors such as GPS.
A recent navigation system, such as disclosed in Japanese Patent Application Publication (KOKAI) No. 2-244188, uses a pseudo three dimensional display method in which a coordinate transformation of two dimensional map information is executed to generate a pseudo three dimensional image so as to display the map information in the vicinity of the current automobile location in a more easily recognizable manner. The pseudo three dimensional display may give a kind of three dimensional impression to a viewer using the two dimensional map information because, in the pseudo three dimensional display, a view point is set at a predetermined position above the automobile, and a perspective image viewed along a viewing direction which makes an acute angle with respect to the ground plane is displayed.
The pseudo three dimensional display of the prior art is generated from the two dimensional map information using the view point determined while assuming a flat ground surface. This may cause some problems if the stereoscopic map display method mentioned above is executed using three dimensional map information including information regarding a topographical elevation, instead of the two dimensional map information.
Namely, the three dimensional map information contains topographical information such as elevations of mountains, buildings or the like. If such three dimensional information is used and the view point is established with respect to the ground plane at sea level (altitude of 0 m) in the same way as that of the pseudo three dimensional display method, the established view point may end up inside a mountain which protrudes above the ground plane, depending on its height. If that is the case, the navigation system may not be able to display a location of the automobile or carry out the navigation procedure.